It is known that aqueous systems, particularly industrial aqueous systems such as boiler systems, cooling tower systems, heat exchanger systems, desalination systems, paper mills, heating/cooling systems, fire service water, reactors, and the like are subject to the formation of deposits on the internal surfaces which are in contact with the circulating water. The removal of scale and iron oxide deposits, which arise either from the chemical reaction of the water with the metal surfaces of the system's piping and other parts, as well as from being present in certain water supplies, is necessary to prevent "under deposit corrosion" of metallic surfaces and for the maintenance of clean heat transfer surfaces to assure good thermal efficiency. A buildup of metal oxide will affect the rate of heat transfer, will cause the pipes to become clogged (limiting flow) and, in general, add to the corrosive aspect of the water. A metal surface which is exposed to a corrosive environment, such as water which contains dissolved oxygen and dissolved ionic inorganic solids will be subject to corrosion under the deposits of solids that form on the metal. If the deposits are not removed, under deposit corrosion can penetrate through the metal, breaching the containment. Once this occurs, fluid starts leaking from the system and the system must be taken off line and this portion of the system must be repaired or replaced.
A number of approaches have been attempted to create an effective rust and/or scale cleaner. The use of chelating agents or chelants are known for their ability to remove particular cations such as iron. Also, chelants tend to be somewhat specific with respect to which cations they will complex with and under what conditions. As disclosed in U.S. Pat. No. 5,183,573, the disclosure of which is incorporated herein in its entirety by reference, chelants function by essentially "locking" the metal into a soluble organic ring structure. The use of chelants, such as ethylene diamine tetraacetic acid (EDTA), along with dispersants is well known for removing iron, because iron has six coordination sites as does EDTA and iron is removed by EDTA since it forms a stable metal chelant with iron. This approach often results in clean waterside surfaces. However, the greatest drawback to this approach is that the reaction that removes oxidized metal is not self-terminating. The chelant will continue to dissolve the surface metal even after the corroded oxide coating is removed. This can lead to a very serious problem, the problem the cleaner was to prevent, i.e. perforation of the metal leading to tube failure or joint leaks. An attempt to solve this problem is disclosed in U.S. Pat. No. 5,171,477, the disclosure of which is incorporated herein in its entirety by reference.
Another approach is using high concentrations of aminoalkylphos-phonates, but these cleaners are expensive and leave a fresh metal surface which "flash rusts" leaving a thin film of the very oxide which was just removed. Disposal also becomes a problem with these cleaners in areas where phosphate discharge limits are legislated. The use of a carboxyalkyl, aminoalkyl hydroxyaryl sulfonic acid is also available, but this cleaner requires long time periods, i.e. weeks, to be effective. Organic compounds, such as hydroquinone and quinone have been used as iron oxide dispersants at high doses, but their cost is prohibitive and their use is restricted to alkaline pH ranges. Acid cleaners, such as, muriatic acid, sulfamic acid, etc. have been used, but the low pH's at which these agents work is hazardous and also presents disposal problems.
Methods of treating or removing iron contaminants, such as iron oxide, from aqueous systems are also disclosed in the following U.S. Pat. Nos. 3,806,459; 4,190,463; 5,022,926; and 5,223,146.
It would be very desirable to be able to quickly remove scale and/or rust deposits from metal surfaces under safe conditions without the use of strong acids. It would also be desirable to be able to remove scale and/or rust deposits from metal surfaces without the risk of perforating the metal wall of the system. Further, it would be desirable to be able to remove scale and/or rust deposits from metal surfaces while leaving a stable protective coating.